Back to EveryPatent.com
| United States Patent |
5,287,218
|
|
Chen
|
February 15, 1994
|
Re-imaging optical system including refractive and diffractive optical
elements
Abstract
A re-imaging optical system (10) has a ref lective objective (12) providing
an intermediate image of the object being viewed and a relay (14)
including refractive (32, 62) and diffractive (34, 72) optical elements.
The system is capable of re-imaging the intermediate image onto an image
plane (16) with the characteristic advantages of reflective and refractive
systems while eliminating their deficiencies.
| Inventors:
|
Chen; Chungte W. (Irvine, CA)
|
| Assignee:
|
Hughes Aircraft Company (Los Angeles, CA)
|
| Appl. No.:
|
864858 |
| Filed:
|
April 7, 1992 |
| Current U.S. Class: |
359/365; 359/434; 359/565; 359/569; 359/858 |
| Intern'l Class: |
G02B 017/08; G02B 027/44; G02B 005/10 |
| Field of Search: |
359/362,364,365,399,423,434,558,565,566,569,724,726,727,730,857,858,861,864
|
References Cited
U.S. Patent Documents
| 3897133 | Jul., 1975 | Warner et al. | 359/365.
|
| 4265510 | May., 1981 | Cook | 359/366.
|
| 4737021 | Apr., 1988 | Korsch | 359/366.
|
| 4804258 | Feb., 1989 | Kebo | 359/366.
|
| 4834517 | May., 1989 | Cook | 359/366.
|
| 4895790 | Jan., 1990 | Swanson et al. | 359/569.
|
| 4964706 | Oct., 1990 | Cook | 359/366.
|
| 4993818 | Feb., 1991 | Cook | 359/366.
|
| 5009494 | Apr., 1991 | Iossi et al. | 359/366.
|
| 5044706 | Sep., 1991 | Chen | 359/571.
|
| 5153772 | Oct., 1992 | Kathman et al. | 359/741.
|
| Foreign Patent Documents |
| 0156737 | Jul., 1986 | JP | 359/365.
|
| 91/12551 | Aug., 1991 | WO | 359/565.
|
| 0476147 | Dec., 1937 | GB | 359/365.
|
Other References
Swanson, G. J. "Binary Optics Technology: The Theory and Design of
Multilevel Diffractive Optical Elements" Technical Report 854, M.I.T.
Lincoln Laboratory, Aug. 14, 1989.
Hecht, Eugene Optics, 2nd Edition, Addison-Wesley Publishing Co. Reading
MA, 1987 pp. 188-190, 196-198.
|
Primary Examiner: Arnold; Bruce Y.
Assistant Examiner: Parsons; David R.
Attorney, Agent or Firm: Gortler; Hugh P., Sales; Michael W., Denson-Low; W. K.
Claims
What is claimed is:
1. A re-imaging optical system comprising:
an objective including a primary and secondary mirror, said primary and
secondary mirrors having conic or higher order aspheric surfaces, said
primary mirror receiving and projecting a radiation beam of a viewed
object, said secondary mirror positioned to receive and reflect said
radiation beam from said primary mirror, said primary mirror forming an
intermediate image of said viewed object;
a relay means including a refractive optical element and a diffractive
optical element positioned to receive said radiation beam from said
secondary mirror to re-image said intermediate image onto an image plane.
2. The system according to claim 1 wherein a fold mirror is positioned
between said primary and secondary mirrors for relaying said radiation
beam to said secondary mirror.
3. The system according to claim 1 wherein said primary mirror and said
secondary mirror are each positive power mirrors.
4. The ssytem according to claim 1 wherein said refractive and diffractive
optical elements have positive optical power.
5. The system according to claim 1 wherein said refractive and diffractive
optical elements are a single hybrid optical element.
6. The system according to claim 1 wherein said refractive optical element
includes a pluraliyt of lenses.
7. The system according to claim 6 wherein said plurality of lenses
includes positive and negative power lenses, the overall power of said
plurality of lenses being positive.
8. The system according to claim 7 wherein an exit pupil is positioned
between said secondary mirror and said plurality of lenses.
9. The system according to claim 6 wherein the combination of a group of
crown-flint lenses and a refractive-diffractive hybrid optical element
corrects primary and the secondary axial chromatic aberrations.
10. The system according to claim 6 wherein the combination of a group of
crown-flint lenses and a refractive-diffractive hybrid optical element
corrects primary and secondary lateral chromatic aberrations.
11. The system according to claim 1 wherein an exit pupil is positioned
between said imaging plane and said diffractive optical element.
Description
BACKGROUND OF THE INVENTION
1. Technical Field
This invention relates to re-imaging optical systems and, more
particularly, to a catatrioptric re-imaging optical system utilizing
reflective, refractive, and diffractive optical elements. 2. Discussion
Re-imaging optical systems generally include two optical modules, an
objective group and a relay group. The objective group precedes the
intermediate image plane and the relay group follows the intermediate
image. Re-imaging optical systems are key optical components in many
optical sensors. Re-imaging optical systems also have unique properties.
Some of their properties are better rejection of off axis radiation, one
hundred percent cold shielding with the cold stop right next to the
detector array, and an accessible entrance pupil.
Two examples of common re-imaging optical systems are all reflective three
mirror anastigmats and catadioptric optical systems. Three mirror
anastigmatic systems generally include two concave mirrors and one convex
mirror. Therefore, two of the mirrors are positive and one is negative
power. The advantage of a three mirror anastigmat is that it is of a
simple optical design which has substantially no chromatic aberration and
is generally, relatively speaking, less expensive to fabricate when
compared to refractive optical systems. Unfortunately, it is difficult to
avoid vignetting or central obscuration without jeopardizing the field of
view coverage. The vignetting problem is particularly severe for the
tertiary mirror. In order to clarify both the exit pupil and image planes
from radiation reflected off the secondary mirror, before impinging on the
tertiary mirror, either the line of sight is offset from the optical axis
or the tertiary mirror is purposely tilted. Thus, the total usable field
of view is limited.
While a catadioptric optical system experiences less vignetting and
obscuration problems, they are generally more complicated. The
complication is due to the requirement of chromatic aberration correction.
To alleviate the chromatic aberration problem, the optical power of each
refractive optical element is reduced. In many cases, the optical power of
the refractive optical element group is insignificant as compared to that
of the reflective optical element.
Thus, three mirror anastigmatic optical systems exhibit vignetting and
observation problems, while catadioptric optical systems are complicated
due to the requirements for chromatic aberration correction. The present
invention provides an optical system which overcomes the above problems.
SUMMARY OF THE INVENTION
According to the teachings of the present invention, a system is provided
which maintains favorable characteristics of reflective and refractive
optical systems, while eliminating their disadvantages. The present
invention provides an optical system including reflective, refractive and
diffractive optical elements. This system not only preserves the
advantages of reflective optical systems, such as wide field of view,
broad spectral band width and low cost, but also has the advantage of
catadioptric optical systems, such as compactness and long working
distances to simplify focal plane assembly interface.
The present invention combines reflective, refractive and diffractive
optical elements into a re-imaging optical system which maintains the f
avorable characteristics of each individual system while avoiding their
disadvantages. The present system may be easily manufactured and is
capable of wide field of view, wide spectral band width and is very
compact and inexpensive. Thus, optical sensors constructed according to
the teachings of the present invention are compact, have better image
quality, and are easier to package and less expensive.
In the preferred embodiment, the re-imaging optical system includes first
and second optical modules. The first module is an objective with a
primary mirror. The primary mirror forms a radiant beam which includes an
image of the object being viewed. The objective forms an intermediate
image of the object being viewed. The second module is a relay which
includes a secondary mirror and a refractive and diffractive optical
element. The radiant beam received from the secondary mirror is passed
through the refractive and diffractive optical elements to re-image the
intermediate image onto an imaging plane.
BRIEF DESCRIPTION OF THE DRAWINGS
The various advantages of the present invention will become apparent to
those skilled in the art after a study of the following specification and
by reference to the drawings in which:
FIG. 1 is a diagrammatic side elevation view of an optical system in
accordance with the teaching of the present invention.
FIG. 2 is a diagrammatic top elevation view of the optical system of FIG.
1.
FIG. 3 is a schematic view of H-tanU curves of the optical systems of FIGS.
1 and 2.
FIG. 4 is a schematic view of H-tanU curves of an optical system like that
of FIGS. 1 and 2 without the diffractive optical element for chromatic
aberration correction.
FIG. 5 is a diagrammatic side elevation view of a second embodiment of the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIGS. 1 and 2, a re-imaging optical system is shown and
designated with the reference numeral 10. The optical system includes an
objective group 12 and a relay group 14 focusing the radiation beam of an
object being viewed onto an image plane 16. Generally the radiation enters
an entrance pupil 18 and exits an exit pupil 20, which is positioned
between the relay group 14 and image plane 16.
The objective group 12 generally includes a primary mirror 22, a secondary
mirror 24 and a fold mirror 26. The primary mirror 22 includes a central
axis which defines the system optical axis 28. The primary mirror 22 is a
positive power mirror and may be a conic or higher order aspheric mirror.
The primary mirror 22 forms an intermediate image of the object being
viewed.
The secondary mirror 24 is a positive power mirror and is positioned on
axis with respect to the optical axis 28. The secondary mirror is a conic
or higher order aspheric mirror.
The fold mirror 26 is a planar mirror. The fold mirror is positioned in the
optical path of the beam 30 to fold the beam to make the system more
compact.
The relay group 14 includes a refractive optical element 32 and a
diffractive optical element 34. In FIGS. 1 and 2, the refractive and
diffractive optical elements are a hybrid optical element, thus, a single
element includes both a refractive and diffractive optical element. The
refractive and diffractive optical elements are tilted 8.667 degrees and
decentered 1.2077 inches with respect to the optical axis 28. In some
applications, the hybrid optical element can be positioned on axis with
respect to the optical axis.
The radiant beam 30 is received and reflected from the secondary mirror 24
and projected to the refractive and diffractive elements 32 and 34. As the
beam passes through the refractive 32 and diffractive 34 elements, the
intermediate image is re-imaged and passed to the image plane 16. The
field curvature introduced by the primary 22 and secondary 24 mirrors is
balanced by the refractive optical element. The refractive and diffractive
optical elements both have a positive optical power. Thus, the positive
optical power of all the optical elements provides for field curvature
correction, reducing the overall size of the unit and also reducing the
complexity of the system.
FIG. 3 is the H-tanU curves of the optical system shown in FIG. 1. H-tanU
curves have been used by skilled optical designers to describe the
geometric aberration of an optical system. Those curves in the left and
right hand sides correspond to the tangential and sagittal geometric
aberrations, respectively. The top, middle and bottom curves are the
geometric aberrations at full field, 70% field and on-axis, respectively.
The curves 1, 2 and 3 in each H-tanU plot are the geometric aberrations
for the wavelengths of 3.8 .mu.m, 3.6 .mu.m and 4.2 .mu.m, respectively.
The wider the spread among the three colors, the worse the chromatic
aberration. The H-tanu curves in FIG. 3 show practically no chromatic
aberration. The chromatic aberration introduced by the refractive optical
element is balanced out by the diffractive optical element. FIG. 4 is the
H-tanU curves of a similar optical system except a diffractive optical
element is not used on element 32 to correct the chromatic aberration.
FIG. 4 shows a significant amount of chromatic aberration.
A specific prescription for a re-imaging optical system having the
configuration illustrated in FIGS. 1 and 2 is given in the following
table.
TABLE 1
__________________________________________________________________________
Radius Decenter
Tilt Thickness
Element
(inches)
CC (inches)
(degrees)
(inches)
__________________________________________________________________________
Primary
-2.91128
<21.334
0 0 1.45564
Mirror
AD = -0.31987E-1 AE = 0.12248E-1
Secondary
-5.01035
0.13114
0 0 2.18400
Mirror
AD = -0.35941E-2 AE = 0.34629E-2
Fold .infin. -2.60000
Mirror
__________________________________________________________________________
Radius of Distance/
Aperture
Curvature Thickness
Diameter
Element
Glass Type
Front/Back (inches)
Front/Back
__________________________________________________________________________
Refractive
silicon
-0.76910/-1.06629
0.1500 0.9/0.9
Element
Diffractive
.function.(.rho.) = 13.9906.rho..sup.2 + 103.993.rho..sup.4
Element
__________________________________________________________________________
AD and AE are the fourth order and sixth order aspheric coefficients
.function.(.rho.) is the grating phase equation of this diffractive
optical element the n.sup.th grating ring boundary is located where .rho
satisfies .function.(.rho.) = n.
.rho. is the radial coordinate
(+ ) Radii have centers to the right
(-) Radii have centers to the left
(+) Thickness to the right
(+) Decenters are up
(+) Tilts are counterclockwise and in degrees
Decenters performed before tilts CC = -.epsilon..sup.2 =
-(Eccentricity).sup.2
Dimensions are given in inches
Reference Wavelength = 3.8 .mu.m
Spectral Range = 0.8 .mu.m
It should be noted that the above prescription is an example for
illustreative purposes and should not be consytrued in any way to limit
the present invention.
Turning to FIG. 5, another embodiment of the present invention is shown.
In FIG. 5, the primary and secondary mirrors 22 and 24 are like those
previously described. However, the fold mirror has been eliminated. Thus,
the beam 30 is reflected directly from the primary mirror 22 to the
secondary mirror 24. Also, an intermediate image is formed between the
primary and secondary mirrors.
Turning to the relay group 14, the refractive optical element group 50 and
refractive-diffractive hybrid optical element 52 are different. Generally,
this arrangement of refractive and hybrid optical elements is utilized for
visible spectral band wavelengths.
The refractive optical element 50 includes a group of four refractive
lenses. The lens group generally includes at least two different types of
nominal glass materials, crown glass and flint glass. The lenses with
positive optical power are crown glass, and the lenses with negative
optical power are generally flint glass. The primary axial chromatic
aberration introduced by the positive optical power lenses is balanced out
by the combination of the negative flint glass and the diffractive optical
element. The secondary axial chromatic aberration is corrected by
balancing the optical power between the flint glass optical elements and
the diffractive optical element. Since the primary axial and secondary
axial chromatic aberrations are very well corrected, the stop shift
introduced primarily lateral and secondary lateral chromatic aberrations
are very small.
The refractive optical element illustrated includes lens 54, lens 56, lens
58 and lens 60.
Lens 54 defines an optical axis which runs through the vertex of the lens.
Generally, lens 54 is a concave-convex lens formed from a crown glass
material. Lens 54 has a predetermined radius of curvature on the concave
or front surface of the lens and a predetermined radius of curvature on
the convex or back surface of the lens. Also, lens 54 has a predetermined
thickness at the vertex and predetermined aperture size on the concave
front surface and the convex back surface.
Lens 56 is centered with respect to the optical axis. Generally this lens
is a convex-concave lens formed from flint glass material. The lens 56 has
a predetermined rate of curvature on the convex or f ront surface of the
lens and a predetermined radius of curvature on the concave or back surf
ace of the lens. The lens 56 has a predetermined thickness at its vertex
and predetermined aperture sizes on the convex front and concave back
surfaces.
The lens 58 is centered with respect to the optical axis. Generally the
lens 58 is a biconvex lens formed from crown glass material. Generally the
lens 58 has a predetermined radius of curvature on the convex or front
surface of the lens and a predetermined radius of curvature on the convex
or back surface of the lens. The lens 58 has a predetermined thickness at
its vertex and predetermined aperture sizes on the convex front and convex
back surfaces. Generally the lens 56 and lens 58 are adhered together by
conventional means to form a doublet.
The lens 60 is centered with respect to the optical axis. Generally, the
lens 60 is a concave-convex lens formed from crown glass material. The
lens 60 has a predetermined radius of curvature on the concave or front
surface of the lens and a predetermined rate of curvature on the convex or
back surface of the lens. Also, the lens 60 has a predetermined thickness
at the vertex and a predetermined aperture size on the concave front
surface and the convex back surface.
The hybrid optical element 52 is centered with respect to the optical axis.
The hybrid optical element 52 consists of a refractive optical element 62
and a diffractive optical element 72. Generally, the refractive optical
element 62 is a convex-planar lens. Generally, the refractive optical
element 62 has a predetermined radius of curvature on the convex or front
surface. The refractive optical element 62 has a predetermined thickness
at its vertex and predetermined aperture sizes on the convex front and
planar back surfaces.
The diffractive optical element 72 is a zone plate pattern with the centers
of the rings generally coinciding with the optical axis of the refractive
optical element. The diffractive optical element, the zone plate pattern,
is imprinted on the second surface of the refractive optical element 62.
Although the second surface of the refractive optical element 62 can be
either concave, convex or flat, a flat surface is generally preferred to
simplify the fabrication processes of the diffractive optical element.
The exit pupil 20 is positioned between the secondary mirror 24 and the
relay group 14. A specific prescription for a re-imaging optical system of
FIG. 5 is given in the following table.
TABLE 2
__________________________________________________________________________
Radius Decenter
Tilt Thickness
Element
(inches)
CC (inches)
(degrees)
(inches)
__________________________________________________________________________
Primary
-12.200
-1.000
0.75 7.500
Mirror
Secondary
2.800 -1.000
0.75 4.900
__________________________________________________________________________
Radius of Distance/
Aperture
Curvature Thickness
Diameter
Element
Glass Type
Front/Back (inches)
Front/Back
__________________________________________________________________________
Lens 54
BAK1 -0.667328/-0.799550
0.250/0.036
0.8/0.96
Lens 56
SF56 4.17351/1.32472
0.120/0.00
1.0/1.0
Lens 58
LAK9 1.32471/-2.76912
0.380/0.065
1.0/1.0
Lens 60
LAKN7 -1.90347/-2.68732
0.320/0.0198
1.0/1.0
Lens 62
LAKN7 1.69427/.infin.
0.394/0.00
1.0/1.0
Diffractive
.function.(.rho.) = 269.81477.rho..sup.2 - 22.83324.rho..sup.4
Element
__________________________________________________________________________
.function.(.rho.) is the grating phase equation of this diffractive
optical element the n.sup.th grating ring boundary is located where .rho
satisfies .function.(.rho.) = n
.rho. is the radial coordinate
(+) Radii have centers to the right
(-) Radii have centers to the left
(+) Thickness to the right
(+) Decenters are up
(+) Tilts are counterclockwise and in degrees
Decenters performed before tilts CC = -.epsilon..sup.2 =
-(Eccentricity).sup.2
Dimensions are given in inches
Reference Wavelength = 0.75 .mu.m
Spectral Range = 0.30 .mu.m
It should be noted that the above prescription is an example for
illustrative purposes and should not be construed in any way to limit the
present invention.
The advantages provided by the examples of the preferred embodiment of the
invention include the utilization of both reflective and refractive
optical systems which provides their collective advantages and limits
their deficiency. Thus, sensors constructed according to the teachings of
the present invention are more compact, have better image quality and are
easier to package while being relatively less expensive.
While it is apparent that the preferred embodiment is well calculated to
fulfill the above stated objects, it will be appreciated that the present
invention is susceptible to modification, variation and alteration without
varying from the proper scope and fair meaning of the subjoined claims.
Top